This invention relates to heat exchangers, and more particularly, to a lanced and offset fin of the type used within a fluid flow path within a heat exchanger.
Many heat exchangers in use today employ one or more rows of so-called flattened tubes which extend between headers provided with tanks or even tubular headers. External fins are bonded to the exterior of the flattened tubes and in some instances, internal fins within the tubes are utilized. Such fins increase surface area within the tubes and provide a means whereby heat may flow from the fluid flowing within the tubes to the insert and then to the walls of the tube through the insert. Thus, where the insert is a better conductor of heat than the fluid flowing within the tube, enhanced heat transfer results.
In addition, such inserts may provide a turbulating function. That is to say, they increase turbulence in the fluid flowing within the tube which in turn is known to increase heat transfer efficiency.
Moreover, where such tubes are to carry fluid at a relatively high pressure and are not supported by the external fins, the inserts, being bonded to both side walls, strengthen the tubes as well.
While inserts of the sort generally alluded to previously have taken on many forms, a so-called xe2x80x9clanced and offsetxe2x80x9d fin is preferred in many applications. The heat exchanger art is beginning to recognize that lanced and offset fins are xe2x80x9cthexe2x80x9d internal fin for use in flattened tubes because of their ability to perform all of the above functions with at least the same efficacy, and in many instances greater efficacy, than more standard internal fin configurations. Notwithstanding, there has not been universal adoption of lanced and offset fins for such applications.
In particular, heretofore, lanced and offset fins have been produced by what the art refers to as stitching machines. In the operation of such machines, the dies that produce the lanced and offset configuration of the fin move forward and back and from side to side. The fin formed has a flow path that extends in the direction across the stitching machine. Thus, the length of the fin is limited to the maximum operative width of the stitching machine. As a consequence, and dependent upon the length of the flattened tubes that are to be provided with such lanced and offset fins, it may be necessary to insert the lanced and offset fin as more than one piece in order to extend for the full length of the flattened tube. Unfortunately, this takes plural insertion operations which are time consuming and when more than one fin piece is inserted into a tube, there is a possibility that there will be a gap between the insert pieces. At such a location, there will be no insert to bond to the interior sides of the tube and as a consequence, there will be a location that is not provided with enhanced strength by the presence of an insert bonded thereto. Consequently, the possibility of failure when subject to high pressure is enhanced.
Furthermore, the very nature of the stitching machine operation is such that it is a very, very slow production method. Typically, for a length equal to the maximum operative length of the stitching machine, the stitching machine can only produce one leg of a lanced and offset fin during each second of operation. Thus, a fin having six legs would require six seconds to manufacture.
Furthermore, stitched inserts have a tendency to nest in one another, making them difficult to separate during production. The fins may be damaged during the separation process and require scrapping for this reason. Alternatively, if they cannot be readily separated, an assembly of two or more nested fins may require scrapping because they cannot be separated.
The present invention is directed to overcoming one or more of the above problems.
It is the principal object of the invention to provide a new and improved method for making a lanced and offset fin. More specifically, it is an object of the invention to provide a method of making such a fin in a continuous fashion without the need for the use of stitching machines. In a preferred embodiment, the invention contemplates a method of making a lanced and offset fin with roll forming equipment.
An exemplary embodiment of the invention contemplates a method of making a lanced and offset heat transfer fin having xe2x80x9cnxe2x80x9d legs and which includes the steps of: a) advancing an elongated, generally planar strip of fin forming material in its direction of elongation along a fin forming path; b) forming xe2x80x9cnxe2x80x9d rows of transfer slits in the strip with crowns extending between adjacent slits in each row at locations intermediate the ends of the slits with adjacent crowns in each row being oppositely directed; and c) thereafter bending the strip through a substantial acute angle at each crown and at the ends of adjacent slits in each of the rows with the ends at which the bending occurs alternating from one side of the row to the other for adjacent slits.
In a preferred embodiment, the substantial acute angle is about 90xc2x0.
Most preferably, step c) of the method is performed in at least two sequential operations. According to this embodiment, a first of the sequential operations includes bending to an acute angle substantially less than the substantial acute angle and thereafter bending to the substantial acute angle.
One embodiment of the invention contemplates that xe2x80x9cnxe2x80x9d is an even integer of four or more and one of the operations includes first bending at a first selected two of the rows and the other of the operation includes thereafter bending at a selected different two of the rows.
In a highly preferred embodiment, the first selected two of the rows are the two centrally located rows.
One embodiment of the invention contemplates that step c) is followed by the step of sizing the legs.
In a highly preferred embodiment, steps b) and c) are performed by using at least one roll in each of the steps to form the crowns and transverse slits and to thereafter bend the strips.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.